fix crash caused by trying to unregister null ports
add basic display of existing MIDI ports to audio/MIDI setup
remove JACK related menu items (audio/MIDI setup is where this happens now)
change return type on {disconnect_from,reconnect_to}_engine() and make public so that we can use these when doing intentional disconnects without popping up the "HALTED" dialog
remove JACK related action definitions (audio/MIDI setup is where this happens now)
substantive changes to allow the audio/MIDI setup dialog to change settings as intended

So far only tested with JACK backend, but expected to be more correct for other backends too
minor code changes
more excision of "JACK" from labels, menu items, etc.

NOTE: users will lose the sample rate/latency display in the status bar display. Not yet sure what to do about this
show a different audio/MIDI option screen if JACK is already running. try to fix problem with using AudioBackend::<property-value>() functions before anything has been set.
missing initialization
reallow changing bufsize AND sample rate when we are in control, because this is cool
switch from JACK_Slave to Engine_Slave
changes to integrate new/recent/browse in new session dialog.

Probably some errors/mistakes in here and no doubt some improvements to be done.
rather hacky fix for the change of the SyncSource > JACK enum to SyncSource > Engine

This is needed in order to get ardour started if ardour.rc names "JACK" as the sync source
Merge branch 'master' of https://github.com/nmains/ardour
move advanced new session options into an expander, change some wording
Revert "actually load new state after creating a session."

This reverts commit 914ba475de48f72e57bb6ac8a81888ad71cea7fa.

Unclear why I applied this or what it was supposed to do. A new session has no
state.
if new session button clicked, clear any selection in recent sessions and mark browser unused
likely fix for creating new sessions from templates
reset the visibility of the template selector stuff every time the startup/NSD gets mapped, so that it is always current